In wireless CDMA systems such as those proposed for next generation mobile wireless standards, the major impediments at the physical layer are multipath fading and multiple access interference caused by co-channel users which are not orthogonal to the desired user. The rake receiver, described in R. Price and P. E. Green, "A Communication Technique for Multipath Channels," Proceedings of the IRE, Vol. 46, pp. 555-570, March 1958, attempts to combat multipath fading by coherently combining resolvable multipath replicas of the desired signal. Multiuser detection described in S. Verdu, "Multiuser Detection," Cambridge University Press, New York, 1998, addresses the problem of MAI by actively accounting for its presence when detecting the desired user.
More recently, there has been growing interest in using array processing for further improving receiver performance. These techniques have focused on using multiple antennas at the base station receiver to provide antenna gain and/or diversity gain and allow the possibility of spatial processing. By combining these space-domain techniques with time domain techniques like rake detection and multiuser detection, the resulting space-time detectors show promise of improving the capacity of CDMA systems as compared to traditional time-domain-only detectors. See, for example, A. Paulraj and C. Papadias, "Space-Time Processing for Wireless Communications," IEEE Signal Processing Magazine, Vol. 14, No. 6, pp. 49-83, November 1997. The first generation of space-time CDMA detectors used array processing with either rake detection or multi-user detection. See, respectively, A. Naguib and A. Paulraj, "Performance of Wireless CDMA with M-ary Orthogonal Modulation and Cell Site Antenna Arrays," IEEE Journal on Selected Areas in Communications, Vol. 14, No. 9, pp. 1770-1783, December 1996 or S. Miller and S. Schwartz, "Integrated Spatial-Temporal Detectors for Asynchronous Gaussian Multiple-Access Channels," IEEE Transactions on Communications, Vol. 43, No. 2/3/4, pp. 396-411, February/March/April 1995. Later space-time CDMA detectors combined all three processing techniques. See: H. Huang; S. Schwartz, S. Verdu, "Combined Multipath and Spatial Resolution for Multiuser Detection: Potentials and Problems," Proceedings of the IEEE International Symposium on Information Theory, p. 380, 1995; or M. Nagatsuka and R. Kohno, "A Spatially and Temporally Optimal Multi-User Receiver Using an Array Antenna for DS/CDMA," IEICE Transactions on Communications, Vol. E78-B, No. 11, pp. 1489-1497, November 1995.
While the foregoing systems operate satisfactorily, improvements can be made, in particular to the space-time detectors which combine all three processing techniques. While the detector in the Nagatsuka and Kohno paper is optimum in the maximum likelihood sense, its computational complexity is exponential with respect to the number of users. Hence it is too complex to implement for practical systems. In the paper by Huang, Schwartz and Verdu, a tradeoff between performance and complexity is made, but this detector was not implemented adaptively since it used a zero-forcing criteria. Adaptive implementations allow receivers to account for unknown sources of interference thus improving the detector performance and increasing the system capacity. For example, a base station receiver could account for interference from adjacent cells or from an embedded microcell, while a handset receiver could account for interference from signals it is not explicitly demodulating.